Scoreatack x2 will give you the GL, Arcadefight too. So from there, aim and shoot. You do more damage so you should be able to use the EMP function to get a closer explosion and still kill them.

A Simple Spree - Earn any spree in Firefight Matchmaking - 2000cR

A killing Spree at 10 kills. That' all you need to do.

The House of Blue Leaves - Kill 111 enemies in a single Firefight Matchmaking game on Heroic - 2000cR

Easy enough, Scoreattack will give you 119 enemies in Gruntpocolypse, most other gametypes give you a few more to kill. If you need to get a bunch more, Doubles firefight and Arcade fight will set you well.